1. Field of the Invention
This invention relates to a sheet transfer system, and more particularly to a sheet transfer system for transferring sheets, on which an image is recorded by an image forming system such as a printer or a copier, to a tray.
2. Description of the Related Art
FIG. 9 shows a typical printer. A paper supply table 2 is disposed on one side of a printer body 1 and printing papers 3 are stacked on the paper supply table 2. The printing papers 3 are taken in the printer body 1 by an intake roller 4 and only the uppermost printing paper in the stack is separated from the stack and fed into the printer body 1 by a pair of separator rollers 5. The printing paper 3 thus supplied to the printer body 1 is passed through a resist roller 6 and an ink transfer drum 7, whereby an image is recorded on the printing paper 3 by stencil printing. The printing paper 3 bearing thereon a printed image is transferred by a sheet transfer system 8 and discharged to a tray 9 on the other side of the printer body 1.
In the case of a printer, since the printed paper is still wet with ink, the printed paper is generally transferred by use of a vacuum conveyor (a suction belt) not to contact the front side of the printed paper bearing thereon the printed image. As is well known in the art, the vacuum conveyor generally comprises a pair of conveyor belts 8a which extend in parallel to the direction of transfer on opposite sides of the path along which the printed papers are transferred, and a vacuum fan 8b disposed between the conveyor belts 8a below them. When the vacuum fan 8b is operated to generate suction force, the printed paper is attracted against the conveyor belts 8a under the suction force and when the conveyor belts 8a are driven, the printed paper is conveyed and discharged onto the tray 9.
Conventionally the vacuum fan 8b is disposed at the middle of the conveyor belts 8a as seen in the longitudinal direction thereof. This is for generating the suction force as uniform as possible over the entire area of the transfer area.
However, in fact, the suction force is most strong at the middle of the conveyor belts 8a where the vacuum fan 8b is disposed and is gradually reduced toward the ends of the conveyor belts 8a. Further when the area of the part of the printed paper on which the suction force from the vacuum fan 8b acts is reduced, the attracting force (i.e., the paper holding force) is naturally weakened. When the attracting force is weak, the rotating force of the conveyor belts 8a cannot be efficiently transmitted to the printed paper and the transfer force is weakened.
As a result, when the trailing end of the printed paper passes the middle portion of the transfer path, where the vacuum fan 8b is disposed, the printed paper comes to exist only where the attractive force is weak and the transfer force is abruptly weakened. Further when the leading end portion of the printed paper is transferred beyond the downstream end of the conveyor belts 8a, the area of the part of the printed paper on which the suction force from the vacuum fan 8b acts is reduced, which also results in a weak transfer force. When the transfer force is weakened, the printed paper stalls and sometimes cannot be properly discharged. Further there arises a problem that the printed paper cannot be ejected by a desired distance and cannot be positioned in place on the tray so that the printed papers are stacked with their edges aligned with each other.